Here's why:

* Air is made of matter: Air is a mixture of gases, primarily nitrogen and oxygen. Matter is anything that has mass and takes up space.

* Mass is a fundamental property: Mass is a measure of the amount of matter in an object. Even tiny amounts of matter, like the air inside a ball, still have mass.

* You can feel it: If you've ever bounced a basketball, you've felt the effect of the air inside. The air acts like a spring, providing resistance and helping the ball bounce.

So, while the mass of the air in a ball is small compared to the mass of the ball itself, it's still a real and measurable quantity.